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Bradyarrhythmias
 
 

Overview

Atrioventricular (AV) block occurs due to delayed conduction of an impulse between the atria or ventricles, due to intrinsic or extrinsic causes.
  • Causes of Atrioventricular (AV) Block

  • Intrinsic

  • Idiopathic degeneration
  • Myocardial infarction
  • Congenital - congenital heart disease, neonatal lupus
  • Infiltrative disease - sarcoidosis, amyloidosis, haemochromatosis
  • Cardiac procedures - valvular surgery, correction of congenital heart disease, catheter ablation
  • Systemic lupus erythematosus
  • Myocarditis
  • Extrinsic

  • Physiologic - sleep, athletes
  • Autonomic - carotid sinus massage, carotid sinus hypersensitivity
  • Drugs - beta blockers, calcium channel blockers, digoxin, adenosine, amiodarone
  • Hyperkalaemia
  • Hypothyroidism
Physiologic and autonomic causes of AV block tend to result in first degree or Mobitz I heart block, while pathologic causes tend to result in Mobitz II, advanced second degree or complete heart block.

1st Degree AV Block

First degree AV block indicates delayed conduction between the atria and the ventricles.
  • Look For

  • Prolonged PR interval >200ms (5mm).
  • 1st Degree AV Block
     

2nd Degree AV Block: Type I Mobitz

Also known as Wenckebach AV block.
Type I second degree AV block occurs when there is progressively delayed AV transmission eventually resulting in a missed QRS complex.
  • Look For

  • Initial normal PR interval with progressive prolongation of PR interval followed by a dropped QRS complex.
  • 2nd Degree AV Block: Type I Mobitz
     

2nd Degree AV Block: Type II Mobitz

Type II second degree AV block occurs when there is an intermittently missed QRS complex due to failure of conduction through the AV node.
  • Look For

  • Constant PR interval with intermittently dropped QRS complexes.
  • 2nd Degree AV Block: Type II Mobitz
     

Advanced 2nd Degree Heart Block

In advanced or ‘high-grade' second degree heart block, multiple beats are blocked and it is difficult to distinguish between Mobitz type I and II.
  • Look For

  • Two or more consecutive P waves without QRS complexes
  • May be in a 3:1, 4:1 or higher pattern
  • Advanced 2nd Degree Heart Block
     

3rd Degree AV Block

Complete atrioventricular block represents complete failure of conduction between the atria and ventricles.
  • Look For

  • Complete dissociation of P waves and QRS complexes.
  • 3rd Degree AV Block
     
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